As Mart has mentioned,its a judgement call for a variety of reasons/merits.
On a motorway,general rule of thumb (good practice) is that everything goes on after a certain speed,as you are expecting too much of m.o.p.
If things go wrong,the occupants of the Police vehicle can massively drop in it...but that's another discussion.
The whistling is the air passing the roof bars btw.

I don't mind when a Chav sounds like they're doing 100mph and go past at 20 but when you hear nothing then next thing your car is shaking and a rocket flies past like this, it is scary sh*t! I thought the whistle was his turbo swearing at him...